1. Field of the Invention
This invention relates to improvements in an ultrasonic measurement method and apparatus for subjecting an object to an ultrasonic transmission and receiving reflected ultrasonic waves from within the object to quantitatively measure the acoustic characteristics of the interior of the object. More particularly, the invention relates to an ultrasonic measurement method and apparatus for improving quantitatively the measurement of the acoustic characteristics of the object by correcting for an adverse effect upon the received ultrasonic waves caused by the transmission sound field characteristic of an ultrasonic transducer used to transmit and receive ultrasonic waves.
2. Description of the Prior Art
Ultrasonic measurement techniques find application widely in such fields as materials testing, sonar and medical diagnosis. In particular, ultrasound scanner systems for medical purposes have recently been developed. The principle of operation of an ultrasound scanner system resides in use of a pulse-echo method and utilizes a phenomenon wherein an ultrasonic pulse transmitted into a living body, which is the object undergoing measurement, is reflected at boundaries where there is a difference in acoustic impedence in the living body. The reflected wave (echo) is received and processed to display a tomograph of the living body by a so-called B-mode method. The echo therefore contains such information as the ultrasonic attenuation in the living body, acoustic impedence and propagation velocity of sound. Despite such a variety of effective information contained in the echo, however, the information being utilized at the present time is solely the amplitude of the echo.
More specifically, on the basis of assuming that the propagation velocity of sound in a living body is constant, attenuation ascribable to ultrasonic propagation in the living body is arbitrarily compensated by a so-called STC (sensitivity time control) circuit or TGC (time gain control) circuit, with the corrected echo signal being luminance-modulated and displayed as a tomograph on a cathode-ray tube. This is referred to as a "B-mode display". Accordingly, the tomograph obtained is nothing more than a qualitative picture of a two-dimensional distribution at a surface where the acoustic impedence in the living body is discontinuous, so that the morphological information relating to the position and shape of the bioligical tissue inevitably forms the core of the information utilized. However, the state of the art is such that information such as that relating to ultrasonic attenuation, which is a characteristic of the biological tissue, is not measured.
Several attempts at attaining attenuation information relating to biological tissue have been reported. However, a problem encountered in all of them is that the results of measurement depend upon the sound field characteristics of an ultrasonic beam transmitted by an ultransonic transducer used in measuring attenuation information, e.g. Specifically, as shown in FIG. 1, a transducer 1 of, e.g., a planar disc-shaped configuration transmits an ultrasonic beam 3 into a medium 2 which is a non-attenuating substance or a substance which exhibits very little attenuation. Here the medium 2 is assumed to be degassed water. If the ultrasonic beam 3 is a so-called pencil beam having a constant beam width and a constant sound pressure irrespective of propagation distance X from the transducer 1, then, even when an attenuating medium 4 is disposed at different distances from the transducer 1, as shown in FIG. 2, the characteristics (beam width, sound pressure) of the ultrasonic beam 3 incident upon the medium 4 will be the same despite the difference in distance. The influence of the sound field characteristics of the transmitted beam on the results of measurement can therefore be avoided. As well known, however, ultrasonic waves encounter an interference or diffraction phenomenon, with the result that the transmission sound field characteristic of, e.g., a disc transducer having a finite aperture defines a complicated pattern or field, as shown in FIGS. 3(a) through 3(c). FIG. 3(a) shows that almost all the ultrasonic energy lies within the limits shown therein. FIG. 3(b) shows that the distribution of the relative intensity (Vx/Vm).sup.2 along the central axis of the beam, where amplitude Vx is sound pressure at a position X and Vm maximum sound pressure along the axis X. FIG. 3(c) shows that the energy distributions of beam section at positions (i) through (viii) indicated in FIG. 3(b). Xmax denotes the last position where maximum amplitude along the X axis occurs. Even for the same attenuating medium 4, therefore, the characteristics of the transmitted ultrasonic beam incident upon the medium differ depending upon the distance X from the transducer 1, as shown in FIGS. 3(a) through 3(c). The unfortunate consequence is the aforementioned problem, namely the fact that the results of measurement depend upon the characteristics of the transmitted sound field.
Attempts to solve this problem have been reported. See, for example, "Quantitative Volume Backscatter Imaging" by Matthew O'Donnell in IEEE Transactions on Sonics and Ultrasonics, Vol. 30, No. 1 (1983), and "A beam Corrected Estimation of the Frequency Dependent Attenuation of Biological Tissue from Backscattered Ultrasound"by M. J. T. M. Cloostermans, et. al., in Ultrasonic Imaging, Vol. 5, (1983). These papers describe attempts to diminish the influence of the transmitted sound field characteristics by using an echo signal received from a reference reflector to normalize an echo signal received from a object over an identical distance. A planar reflector immersed in degassed water is employed as the reference reflector. Using the planar reflector for this purpose is a more or less practical method in view of what is stated in the definition of reflective power given on page 49 of the Handbook of Ultrasonic Technology, published by The Nikkan Kogyo Shimbun Ltd., in which it is mentioned that the sound pressure of a wave reflected from a perfectly reflective infinite plane is taken as a standard. Nonetheless, it cannot be truely said on the basis of these reports that fully satisfactory results are obtained by a normalization method using the sound pressure of waves reflected from such a simple planar reflector in due consideration of the complicated energy distributions, as shown in FIG. 3(c). Moreover, there is still no clear solution as to what kind of reflector should best chosen as a standard reflector in the case of a highly complicated object such as biological tissue.